docs: add note about using cluster without networking

Although the primary use-case for the cluster module is networking, the
module provides a generic master/worker interface that could also be
used if you dont use networking at all. Currently the docs are a bit
ambiguous about this as only the primary use-case is ever mentioned,
this remark should clarify that the cluster module can also be used
without disadvantages if you dont use networking.

Refs: https://github.com/nodejs/help/issues/970a

doc/api/cluster.md

@@ -109,7 +109,8 @@ will be dropped and new connections will be refused. Node.js does not
 automatically manage the number of workers, however. It is the application's
 responsibility to manage the worker pool based on its own needs.
 
-
+Although a primary use case for the `cluster` module is networking, it can
+also be used for other use cases requiring worker processes.
 
 ## Class: Worker
 <!-- YAML